Extract conflict detection to it's own interface.

This PR
 - defines a patch conflict detector interface,
 - extracts implementations of the interface from the
   merginator code, making the merginator code
   independent of --enable_kyaml.
 - injects those implementations into kustomize
   as a function of --enable_kyaml.

So, instead of using different merginators to combine
resmaps, this pr allows the use of a single patch merge
code path that uses different conflict detectors.

So instead of debating how to merge, we're now only
considering whether to warn on conflict detection
in one transformer.

This PR is in service of #3304, eliminating seven
instances where --enable_kyaml was consulted.  These
were cases where conflict detection wasn't an issue
(but merging patches was).

api/internal/conflict/factory.go

@@ -0,0 +1,23 @@
+// Copyright 2020 The Kubernetes Authors.
+// SPDX-License-Identifier: Apache-2.0
+
+package conflict
+
+import (
+	"sigs.k8s.io/kustomize/api/resid"
+	"sigs.k8s.io/kustomize/api/resource"
+)
+
+type cdFactory struct{}
+
+var _ resource.ConflictDetectorFactory = &cdFactory{}
+
+// NewFactory returns a new conflict detector factory.
+func NewFactory() resource.ConflictDetectorFactory {
+	return &cdFactory{}
+}
+
+// New returns an instance of smPatchMergeOnlyDetector.
+func (c cdFactory) New(_ resid.Gvk) (resource.ConflictDetector, error) {
+	return &smPatchMergeOnlyDetector{}, nil
+}

api/internal/conflict/smpatchmergeonlydetector.go

@@ -0,0 +1,28 @@
+// Copyright 2020 The Kubernetes Authors.
+// SPDX-License-Identifier: Apache-2.0
+
+package conflict
+
+import (
+	"sigs.k8s.io/kustomize/api/resource"
+)
+
+// smPatchMergeOnlyDetector ignores conflicts,
+// but does real strategic merge patching.
+// This is part of an effort to eliminate dependence on
+// apimachinery package to allow kustomize integration
+// into kubectl (#2506 and #1500)
+type smPatchMergeOnlyDetector struct{}
+
+var _ resource.ConflictDetector = &smPatchMergeOnlyDetector{}
+
+func (c *smPatchMergeOnlyDetector) HasConflict(
+	_, _ *resource.Resource) (bool, error) {
+	return false, nil
+}
+
+func (c *smPatchMergeOnlyDetector) MergePatches(
+	r, patch *resource.Resource) (*resource.Resource, error) {
+	err := r.ApplySmPatch(patch)
+	return r, err
+}